How the Bird Eye Was Pushed to an Evolutionary Extreme
The eye of a red-and-green macaw, with no blood vessels in sight.
红绿金刚鹦鹉的眼睛，看不到血管。
How can a bird eye work so well without oxygen?
鸟眼在没有氧气的情况下怎么能工作得这么好？
When an optometrist shines a bright light into your eyes, a vast, branching tree sprouts in your field of vision.
当验光师将明亮的光线照射到您的眼睛时，一棵巨大的、枝繁叶茂的树就会在您的视野中发芽。
This is the shadow of blood vessels.
这是血管的影子。
Though we normally can’t perceive them, these vessels always occlude a portion of what we see, and for an important reason.
尽管我们通常无法感知它们，但这些血管总是遮挡我们所看到的一部分，这是一个重要的原因。
They power the retina, a thin layer of nerve tissue in the back of the eye that communicates light signals to the brain.
它们为视网膜提供动力，视网膜是眼睛后部的一层薄薄的神经组织，负责向大脑传达光信号。
The retina is one of the body’s most energetically expensive tissues.
视网膜是人体能量最昂贵的组织之一。
Built from complex networks of sometimes more than 100 different types of neurons, retinal tissue consumes two to three times more energy than the same mass of typical brain tissue.
视网膜组织由有时超过 100 个不同类型的神经元组成的复杂网络构成，其消耗的能量是相同质量的典型脑组织的两到三倍。
That’s why most vertebrate retinas, including our own, are furrowed with dense, branching networks of blood vessels: to deliver oxygen and other ingredients for producing energy.
这就是为什么大多数脊椎动物的视网膜，包括我们自己的视网膜，都布满了密集的分支血管网络：输送氧气和其他产生能量的成分。
But there’s a significant exception to this rule.
但这条规则有一个重要的例外。
Birds have retinas that mostly lack blood vessels.
鸟类的视网膜大多缺乏血管。
This may seem especially strange given birds’ exceptional vision.
考虑到鸟类非凡的视力，这似乎特别奇怪。
The bird retina is “one of the most metabolically active tissues in the animal kingdom, yet it worked with no apparent blood perfusion,” said Christian Damsgaard , an evolutionary physiologist at Aarhus University.
奥尔胡斯大学进化生理学家克里斯蒂安·达姆斯加德 (Christian Damsgaard) 表示，鸟类视网膜是“动物界中代谢最活跃的组织之一，但它在没有明显血液灌注的情况下工作”。
“It was a complete paradox.” For centuries this has puzzled scientists, who figured that the bird retina must obtain oxygen through a unique, undiscovered process.
“这完全是一个悖论。”几个世纪以来，这一直困扰着科学家，他们认为鸟类视网膜必须通过一种独特的、未被发现的过程来获取氧气。
Damsgaard is the lead author of a study, published in the journal Nature in January 2026, that showed for the first time that bird retinas don’t have some unusual adaptation for acquiring oxygen — they survive without it entirely.
Damsgaard 是一项研究的主要作者，该研究于 2026 年 1 月发表在《自然》杂志上，该研究首次表明鸟类视网膜在获取氧气方面没有一些不寻常的适应能力——它们完全没有氧气也能生存。
Instead, to bring energy to the tissue, they use a process called anaerobic glycolysis that is significantly less efficient than oxygen-powered metabolism but gets the job done.
相反，为了给组织带来能量，他们使用了一种称为无氧糖酵解的过程，该过程的效率明显低于氧气驱动的新陈代谢，但可以完成工作。
The evolutionary physiologist Christian Damsgaard measured gas exchange in bird eyes with microsensors.
进化生理学家克里斯蒂安·达姆斯加德（Christian Damsgaard）用微传感器测量了鸟眼中的气体交换。
Surprisingly, the inner retina, a highly active tissue, used no oxygen.
令人惊讶的是，内层视网膜是一种高度活跃的组织，不使用氧气。
By studying how tissues can survive without oxygen, researchers can potentially develop therapeutics to treat conditions of oxygen deprivation, such as strokes.
通过研究组织如何在没有氧气的情况下生存，研究人员有可能开发出治疗中风等缺氧病症的疗法。
More fundamentally, they want to understand the limits of evolution.
更根本的是，他们想了解进化的极限。
“What are the extremes of life?” Damsgaard said.
“人生的极端是什么？”达姆斯加德说道。
“How far can we bend the conditions under which highly metabolically active tissues can actually survive?”
“我们可以在多大程度上改变高度代谢活跃的组织实际上可以生存的条件？”
A bird, he learned, can bend them pretty far.
他了解到，鸟可以将它们弯曲得很远。
Around 3.4 billion years ago, cyanobacteria invented photosynthesis.
大约34亿年前，蓝细菌发明了光合作用。
Slowly at first, then quickly, their newly evolved method of making energy from sunlight succeeded and spread.
他们新开发的从阳光中获取能量的方法先是缓慢地，然后很快地成功并传播开来。
The cells pumped so much oxygen, a by-product of photosynthesis, into the atmosphere that it changed the course of life on Earth.
这些细胞将大量的氧气（光合作用的副产品）泵入大气中，从而改变了地球上生命的进程。
Oxygen molecules make energy production in cells extremely efficient.
氧分子使细胞中的能量生产极其高效。
To extract energy, cells break down a glucose molecule into two pyruvate molecules.
为了提取能量，细胞将葡萄糖分子分解成两个丙酮酸分子。
This process releases two molecules of ATP (adenosine triphosphate), life’s universal energy currency.
这个过程会释放出两个 ATP（三磷酸腺苷）分子，这是生命的通用能量货币。
A cell lacking oxygen can go only this far.
缺乏氧气的细胞只能走到这一步。
Oxygen, however, enables further biochemical reactions that break down pyruvate and produce another 30 molecules of ATP.
然而，氧气可以促进进一步的生化反应，分解丙酮酸并产生另外 30 个 ATP 分子。
In other words, the presence of oxygen makes energy extraction from a single glucose molecule 15 times as efficient, and sometimes more.
换句话说，氧气的存在使得从单个葡萄糖分子中提取能量的效率提高了 15 倍，有时甚至更高。
Birds, such as this alpine chough (in the crow family), use their exceptional vision to hunt, forage, and migrate.
鸟类，例如这种高山山鸦（属于乌鸦家族），利用其非凡的视力来狩猎、觅食和迁徙。
This energetic ability is powered by an inefficient metabolism.
这种精力充沛的能力是由低效的新陈代谢提供动力的。
The energetic advantage of oxygen, through the process of aerobic respiration, was transformative.
通过有氧呼吸过程，氧气的能量优势具有变革性。
Once oxygen imbued the atmosphere, evolution selected for organisms that could use it.
一旦氧气渗透到大气中，进化就会选择能够利用它的生物体。
“We’ve been hooked on 20% [atmospheric] oxygen for millions of years,” said Gary Lewin , a molecular physiologist at the Max Delbrück Center in Berlin.
柏林马克斯德尔布吕克中心的分子生理学家加里·勒温 (Gary Lewin) 表示：“数百万年来，我们一直对 20% 的[大气]氧气着迷。”
This Great Oxidation Event was followed by mass extinction, as organisms using oxygen outcompeted just about everybody else.
这次大氧化事件之后发生了大规模灭绝，因为使用氧气的生物体几乎击败了其他生物。
While some life forms, such as certain bacteria, are adapted to life without oxygen, all complex, multicellular organisms need that energy advantage to survive.
虽然某些生命形式（例如某些细菌）适应了没有氧气的生活，但所有复杂的多细胞生物体都需要能量优势才能生存。
Humans and most other animals can survive with little or no oxygen for several minutes at most.
人类和大多数其他动物在很少或没有氧气的情况下最多可以生存几分钟。
The mammal with the highest known tolerance for low-oxygen conditions is the naked mole rat, which can survive for up to 18 minutes breathing anoxic air in underground burrows.
已知对低氧条件耐受性最高的哺乳动物是裸鼹鼠，它在地下洞穴中呼吸缺氧空气可以存活长达 18 分钟。
A few cold-blooded aquatic creatures, including freshwater turtles and goldfish, can persist in low-oxygen conditions at the bottom of a frozen lake for a year or two.
一些冷血水生生物，包括淡水龟和金鱼，可以在冰冻湖底的低氧条件下生存一两年。
But for most animals, a steady supply of oxygen is a must-have.
但对于大多数动物来说，稳定的氧气供应是必须的。
Without oxygen, a variety of processes shut down — especially in metabolically demanding tissues such as the brain.
没有氧气，多种过程就会停止——尤其是在大脑等新陈代谢要求较高的组织中。
Without that energy, our cells malfunction and die.
如果没有这种能量，我们的细胞就会发生故障并死亡。
Naked mole rats can survive without oxygen for 18 minutes.
裸鼹鼠在没有氧气的情况下可以存活18分钟。
To generate energy without oxygen, they use anaerobic glycolysis fueled by fructose.
为了在没有氧气的情况下产生能量，他们利用果糖推动的无氧糖酵解。
All this is why, in 2019, when Damsgaard learned that bird retinas lack blood vessels, he was confused.
这就是为什么，2019年，当达姆斯加德得知鸟类视网膜缺乏血管时，他感到困惑。
How could this high-energy tissue survive, let alone perform at the level observed in sharp-sighted bird species, without oxygen?
在没有氧气的情况下，这种高能组织如何生存，更不用说达到在视力敏锐的鸟类中观察到的水平了？
He pored over the voluminous research on the subject, all of which pointed at a mysterious structure in the bird eye known as the pecten oculi.
他仔细研究了有关该主题的大量研究，所有这些研究都指向鸟眼中被称为眼梳的神秘结构。
In the 17th century, anatomists first described the unusual organ: It looked like a radiator, comblike, riveted with blood vessels, and with a large surface area.
17世纪，解剖学家首次描述了这种不寻常的器官：它看起来像一个散热器，呈梳状，铆有血管，表面积很大。
In the centuries that followed, researchers debated whether it helps deliver oxygen to retinal tissue in bird eyes.
在接下来的几个世纪里，研究人员一直在争论它是否有助于将氧气输送到鸟眼的视网膜组织。
Damsgaard read about 30 different theories about the pecten oculi’s function based on anatomy alone.
Damsgaard 阅读了大约 30 种关于眼梳功能的不同理论，这些理论仅基于解剖学。
“Nobody had really done direct physiological measurements on this structure,” he said.
“没有人真正对这个结构进行过直接的生理测量，”他说。
“That’s where we came in.”
“这就是我们进来的地方。”
In his lab, which studies the exchange of gases such as oxygen and carbon dioxide between vertebrates and their environments, Damsgaard’s team used microsensors to measure oxygen levels in the retinas of zebra finches, pigeons, and chickens.
达姆斯加德的实验室研究脊椎动物及其环境之间氧气和二氧化碳等气体的交换，在他的实验室中，达姆斯加德的团队使用微型传感器测量斑胸草雀、鸽子和鸡视网膜中的氧气水平。
Indeed, in the inner retina, which completely lacks blood vessels, they found no oxygen.
事实上，在完全没有血管的内部视网膜中，他们没有发现氧气。
(They did measure oxygen in the outer retina, at the back of the eye, which has some blood vessels.)
（他们确实测量了眼睛后部的外视网膜中的氧气，那里有一些血管。）
That was “striking,” Damsgaard said.
达姆斯加德说，这“令人震惊”。
“Half of the retina lives in a chronic state of anoxia, where there’s no oxygen present at all.”
“一半的视网膜处于慢性缺氧状态，根本没有氧气存在。”
[Evolution] takes parts that have existed long before, and it recombines, reinvents, and reshapes.
[进化]将早已存在的部分重新组合、重新发明和重塑。
Karthik Shekhar, University of California, Berkeley
Karthik Shekhar，加州大学伯克利分校
Using spatial transcriptomics, a method that combines cell imaging with RNA sequencing, the researchers mapped which genes were active in different parts of the retinal tissue.
研究人员使用空间转录组学（一种将细胞成像与 RNA 测序相结合的方法）绘制了视网膜组织不同部分中活跃的基因图谱。
Genes associated with typical aerobic respiration were expressed in the outer retina, where there are blood vessels.
与典型有氧呼吸相关的基因在有血管的外视网膜中表达。
In the oxygen-depleted inner retina, only genes associated with anaerobic respiration were active.
在缺氧的视网膜内层中，只有与无氧呼吸相关的基因处于活跃状态。
To trace the paths of nutrients, Damsgaard and his team worked with cancer scientists who are experts on oxygen-free metabolism (tumor cells often use anaerobic glycolysis to make energy).
为了追踪营养物质的路径，达姆斯加德和他的团队与癌症科学家合作，他们是无氧代谢方面的专家（肿瘤细胞通常使用无氧糖酵解来产生能量）。
They found that the inner retina demanded 2.5 times more glucose than other parts of the bird brain.
他们发现，内部视网膜需要的葡萄糖是鸟脑其他部分的 2.5 倍。
Then they examined the pecten oculi.
然后他们检查了眼梳。
Their spatial transcriptomics data showed that the genes for glucose were highly active there.
他们的空间转录组学数据表明，葡萄糖基因在那里高度活跃。
This suggested that the strange structure wasn’t bringing oxygen into the bird’s retina; rather, it was helping to pump glucose in, thereby enabling the less efficient anaerobic process.
这表明这种奇怪的结构并没有将氧气带入鸟类的视网膜；而是将氧气带入鸟类的视网膜。相反，它有助于泵入葡萄糖，从而实现效率较低的无氧过程。
As a by-product, anaerobic glycolysis creates lactic acid, which can accumulate and become toxic.
作为副产品，无氧糖酵解会产生乳酸，乳酸会积累并变得有毒。
The researchers also saw that genes for lactic acid transporters — the molecules that move lactic acid out of tissues — were active in the pecten oculi.
研究人员还发现，乳酸转运蛋白（将乳酸移出组织的分子）的基因在孔梳中很活跃。
The diversity of bird eyes, lacking blood vessels (left to right).
鸟眼的多样性，缺乏血管（从左到右）。
Top: Northern gannet, Eurasian eagle-owl, maguari stork.
上图：北方塘鹅、欧亚雕鸮、马瓜里鹳。
Center: rooster, rockhopper penguin, parrot (species unknown).
中：公鸡、跳岩企鹅、鹦鹉（种类未知）。
Bottom: bald eagle, blue-and-yellow macaw, unknown species.
下：秃鹰、蓝黄金刚鹦鹉，未知物种。
(Left to right) Top: Chris Hellier, Jiří Dočkal, Annette Lozinski.
（从左到右）上：Chris Hellier、Jiří Dočkal、Annette Lozinski。
Center: Mohammed Brzan, Nico Marín, Shyamli Kashyap.
中锋：穆罕默德·布尔赞、尼科·马林、夏姆利·卡什亚普。
Bottom: Ingo Doerrie, David Clode, Hasan Almasi
下：Ingo Doerrie、David Clode、Hasan Almasi
Their findings provide compelling evidence for the role of the pecten oculi in supporting anaerobic glycolysis, which “has been a mystery for a long time,” said Thomas Baden , a neuroscientist at the University of Sussex who was not involved in the study.
他们的发现为梳状眼在支持无氧糖酵解中的作用提供了令人信服的证据，这“长期以来一直是一个谜”，苏塞克斯大学神经科学家托马斯·巴登（Thomas Baden）说，他没有参与这项研究。
“The insight that the retina basically goes oxygen-free, at least in some layers, is surprising.
“视网膜基本上是无氧的，至少在某些层是这样，这一点令人惊讶。
…
……
It really gets properly down to zero.”
它真的会适当地降至零。”
This pathway is used by cancer cells and temporarily by our muscles when they’re strained and can’t get enough oxygen — such as when we’re running.
当我们的肌肉紧张且无法获得足够的氧气时（例如当我们跑步时），癌细胞会使用这条通路，并且我们的肌肉会暂时使用这条通路。
But no known vertebrate tissue was known to survive in completely anoxic conditions for a lifetime.
但目前还没有已知的脊椎动物组织能够在完全缺氧的条件下存活一生。
The bird’s retina and its no-oxygen power system are so unusual that they naturally raise questions about how they could have evolved.
这种鸟的视网膜及其无氧动力系统非常不寻常，自然会引发人们对它们如何进化的疑问。
This is “a series of beautiful experiments,” said Karthik Shekhar of the University of California, Berkeley, who was not involved in the research.
加州大学伯克利分校的卡蒂克·谢卡尔（Karthik Shekhar）没有参与这项研究，他说，这是“一系列美妙的实验”。
It’s an example of how an animal took the vertebrate eye — a highly conserved structure whose origins go back some 560 million years to a light-sensitive patch on a primitive creature — and tinkered with it to fit its own needs.
这是一个动物如何利用脊椎动物眼睛（一种高度保守的结构，其起源可以追溯到大约 5.6 亿年前原始生物身上的感光斑块）的例子，并对其进行修改以适应自己的需要。
“Evolution is not really like an inventor; it acts more like a tinkerer,” he said, citing a 1977 essay, “ Evolution and Tinkering ,” by the French biologist François Jacob.
他引用法国生物学家弗朗索瓦·雅各布 (François Jacob) 1977 年发表的一篇文章《进化与修补》说：“进化其实并不像一位发明家；它的作用更像是一位修补匠。”
“It takes parts that have existed long before, and it recombines, reinvents, and reshapes.”
“它采用了很久以前就存在的部件，然后重新组合、重新发明和重塑。”
The researchers tried to pinpoint when the pecten oculi might have arisen by comparing oxygen levels in the bird retina to those in not-so-distant relatives: two reptile species, Chinese pond turtles and broad-snouted caimans.
研究人员试图通过将鸟类视网膜中的氧气水平与不太远的亲戚（两种爬行动物，中国池塘龟和宽吻凯门鳄）的视网膜中的氧气水平进行比较来查明眼梳可能出现的时间。
The reptile retinas had normal oxygen levels and no indication of anaerobic glycolysis.
爬行动物视网膜的氧含量正常，没有无氧糖酵解的迹象。
This led Damsgaard’s team to conclude that the oxygen-free tissue likely evolved sometime during the dinosaur era, after the avian lineage had split from crocodiles but hadn’t yet evolved into modern birds.
这使得达姆斯加德的团队得出结论，无氧组织可能是在恐龙时代的某个时候进化出来的，当时鸟类谱系已经从鳄鱼中分离出来，但尚未进化成现代鸟类。
This was around the same time that the retina thickened.
大约在同一时间，视网膜变厚。
Still, that rough time estimate can’t explain what evolutionary pressure might have selected for the unusual retinal tissue.
尽管如此，粗略的时间估计仍无法解释进化压力可能选择了不寻常的视网膜组织。
Researchers can only speculate.
研究人员只能推测。
“I think the system evolved in theropod dinosaurs in response to selection for sharp vision for tracking prey and identifying mates,” Damsgaard suggested.
达姆斯加德表示：“我认为兽脚亚目恐龙进化出这个系统是为了响应敏锐的视觉选择，以跟踪猎物和识别配偶。”
Then, later, when birds took to the skies, it “served as the physiological basis for maintaining retinal function” during high-altitude flights, when oxygen levels are low, he speculated.
后来，当鸟类飞上天空时，他推测，在高空飞行期间，当氧气含量较低时，它“成为维持视网膜功能的生理基础”。
The lack of blood vessels could also offer birds the advantage of better vision.
缺乏血管也可以为鸟类提供更好的视力。
The bird retina is complex and densely packed with more than a hundred cell types that work to render the world in great resolution.
鸟类的视网膜非常复杂，并且密集地分布着一百多种细胞类型，这些细胞能够以高分辨率呈现世界。
Birds use their exceptional visual sense for hunting and foraging — consider an owl tracking a mouse from the sky, an albatross watching for signs of fish on the ocean’s surface, or a hummingbird locating hundreds of flowers every day — as well as for following landmarks across the landscape during migration.
鸟类利用其非凡的视觉来狩猎和觅食——想象一下猫头鹰从天上追踪老鼠，信天翁在海面上寻找鱼类的踪迹，或者蜂鸟每天定位数百朵花——以及在迁徙过程中追踪景观中的地标。
Without blood vessels obstructing their view, birds’ retinal cells might be able to take in more visual information.
如果没有血管阻挡鸟类的视野，鸟类的视网膜细胞可能能够吸收更多的视觉信息。
Could this be an adaptation, or is it a coincidence of evolutionary history?
这可能是一种适应，还是进化历史的巧合？
There’s no way to know for sure how birds’ incredible vision evolved.
我们无法确切知道鸟类令人难以置信的视觉是如何进化的。
There’s this mystery “that has lingered around us,” Baden said.
巴登说，这个谜团“一直萦绕在我们周围”。
“What is it about birds that makes their eyes so special?” Their retinal power system seems as if it could explain what makes them so unique.
“鸟类的眼睛有何特别之处？”它们的视网膜能量系统似乎可以解释它们如此独特的原因。
However, Lewin, the physiologist, is cautious about overextending the results and interpretations to every bird, given that the researchers haven’t looked at any migratory species.
然而，鉴于研究人员尚未研究过任何迁徙物种，生理学家勒温对将结果和解释过度延伸到每一种鸟类持谨慎态度。
The implications stretch well beyond bird adaptations to biomedicine.
其影响远远超出了鸟类对生物医学的适应范围。
A common thread in many medical conditions is a drop in oxygen delivery to tissues, which, depending on where it occurs, can lead to scars or brain damage.
许多医疗状况的一个共同点是组织供氧量下降，根据发生部位的不同，可能会导致疤痕或脑损伤。
Human brains can tolerate maybe a minute of total anoxia, Lewin said.
勒温说，人类大脑可能可以忍受一分钟的完全缺氧。
That’s what makes strokes, which cut off blood and oxygen supply to parts of the brain, so devastating.
这就是中风造成如此严重的原因，中风会切断大脑部分区域的血液和氧气供应。
By studying low-oxygen conditions in creatures such as naked mole rats and birds, scientists can gain insight into how tissues can tolerate low-oxygen conditions.
通过研究裸鼹鼠和鸟类等生物的低氧条件，科学家可以深入了解组织如何耐受低氧条件。
“Maybe we can get inspiration for how nature solved these problems by millions of years of natural selection,” Damsgaard said.
“也许我们可以从大自然如何通过数百万年的自然选择解决这些问题中获得灵感，”达姆斯加德说。
“There’s so much to be learned from these animals that are able to do something that we cannot do.”
“从这些动物身上可以学到很多东西，它们能够做我们做不到的事情。”
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辱骂、亵渎、自我推销、误导、语无伦次或偏离主题的评论将被拒绝。
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